the easiest way is to pick one groupset, ie shimano xt or SRAM x7 and use that for the entire drivetrain, i prefer to have x9/x0 shifter derailleur, SRAM cassette and KMC chains, its all personal preference

RANDOM THOUGHT:What's wrong with your 8 speed setup?If it were me, I'd just be replacing the 8 speed (with 8 speed). You can still get XT 8 speed cassettes which are as good as anything else on the market, if that's too spensy go with a deore.8 speed is bulletproof